1. Field of the Invention
The present invention relates to a motor control apparatus and particularly to protection of an inverter used to operate a motor as part of the apparatus.
2. Description of the Prior Art
FIG. 1 is a block diagram of a motor control apparatus of the prior art disclosed, for example, in Japanese Published Patent No. 63-274390 and Japanese Laid-open Patent No. 61-10983. In this figure, the letters R, S, T designate a 3-phase AC power supply. The numeral 2 designates an AC/DC converter connected to the AC power supply R,S,T and is controlled to generate a DC output voltage; 3 is a smoothing capacitor for smoothing an output of the converter 2; 4 is, a recovery control circuit including a resistor and a switch element and connected in parallel with capacitor 3; 5 is a PWM inverter which is connected to both ends of the smoothing capacitor 3 and is formed by transistors and diodes to generate AC outputs 5a to 5c of variable voltage and variable frequency by through pulse width modulation (PWM); 6 is a 3-phase induction motor which is driven by an output of the inverter 5; 7 is, a speed detector which is connected in directly to the motor 6 to generate the speed signal 7a in proportion to the rotational speed of motor 6; 8 is a load which is driven by the motor 6; 12-14 are current transformers for generating current feedback signals 12a-14a corresponding to the primary current of each phase of motor 6; 15 is, a coordinate converting circuit having a 3-phase/2-phase converting circuit for inputting a sine wave signal 21a and cosine wave signal 21b and converting the current feedback signals 12a-14a to an excitation current component signal 15a and torque current component signal 15b on a coordinate axis system which rotates in synchronization with the angular speed of the secondary magnetic flux vector of motor 6; 16 is, a divider; 17 is, a coefficient multiplying circuit for generating a slip frequency signal 17a by multiplying an input signal by a coefficient; is a normal rotation amplifier of a gain P (corresponding to the number of poles of motor 6) to which the speed signal 7a, is inputted; 19 is adder for generating a synchronous angular speed signal 19a by adding the slip frequency signal 17a to an output signal of the normal rotation amplifier 18; 20, is an integrator for generating a phase angle signal 20a of a secondary magnetic flux vector by integrating the synchronous angular speed signal 19a; 21 is a function generator for inputting the phase angle signal 20a and generating a corresponding sine wave signal 21a and a cosine wave signal 21b; 22 is a subtractor for generating an excitation current deviation signal by subtracting the excitation current component signal 15a from an excitation current component command value 23; 24 is an excitation current control circuit which is formed by lag/lead circuit and operates to cause the output of subtractor 22 to become zero; 24a is an excitation voltage component command value; 25 is a subtractor for generating a deviation signal by subtracting the speed signal 7a from a speed command value 26; 27 is a speed control circuit which is formed by a lag/lead circuit and operates to cause the output of the subtractor 25 to become zero; 27a is a torque current component command value; 28 is subtractor for generating a torque current deviation signal by subtracting a torque current component signal 15b from the torque current component command value 27a; 29 is a torque current control circuit which is formed by a lag/lead circuit and operates to cause the output of subtractor 28 to become zero; 29a is a torque voltage component command value; 30 is a coordinate converting circuit having a 2-phase/3-phase converting circuit for inputting sine wave signal 21a and cosine wave signal 21b and converting the excitation voltage component command value 24a and torque voltage component command value 29a into primary voltage command values 30a to 30c of each phase, which are inputted to PWM inverter 5.
The numeral 31 designates a printed circuit board on which the coordinate converting circuit A 15 through AND gate 39 are incorporated a microcomputer which constitutes the coordinate converting circuit A 15 through the coordinate converting circuit B 30; 33 is a connector for connecting conductors of current feedback signals 12a-14a to the printed wiring board 31; 34 is an overcurrent detecting circuit for detecting the current of 3-phase induction motor 6 and cutting off the PWM inverter 5 when an overcurrent flows; 35 is an overcurrent trip signal; 36 is an operation signal; 37 is an alarm signal outputted from the microcomputer 32; 38 is a gate ON signal which becomes active in the case where the motor is to be driven by the PWM inverter 5; 39 is an AND gate for setting the gate ON signal 38 to HIGH only when the overcurrent trip signal 35 is LOW, an operation signal 36 is HIGH and an alarm signal 37 is LOW.
Next, operation of the motor control apparatus of the prior art will be explained hereunder.
In FIG. 1, the 3-phase AC inputs R, S, T are rectified by the converter 2, and a DC voltage smoothed by the smoothing capacitor 3 is applied to the PWM inverter 5.
The PWM inverter 5 performs switching operations based on the primary voltage command values 30a to 30c. Thereby the AC outputs 5a-5c of variable voltages and variable frequencies are applied to the 3-phase induction motor 6 for rotation of the motor.
To start operation, the operation signal 36 is set to a HIGH level, the speed command value 26 is outputted and the AC outputs 5a-5c are applied to the 3-phase induction motor 6 to start motor rotation.
In this case, the excitation current component signal 15a is controlled to be matched with the excitation current component command value 23.
Moreover, the torque current component signal 15b is also controlled, in the same way, to be matched with the torque current component command value 27a. When the speed is to be lowered, such control is carried out that the speed command value 26 is reduced, the 3-phase induction motor 6 lowers its speed and the speed command value 26 is matched with the speed signal 7a. When the operation signal 36 is made LOW the AC outputs 5a-5c become OFF and the 3-phase induction motor 6 stops.
Since the motor control apparatus of the prior art is constituted as explained above, if connector 33 is disconnected, for example, the current feedback signals 12a-14a are no longer applied to the coordinate converting circuit A 15 and thereby the excitation current component signal 15a and torque current component signal 15b become zero. Therefore, since control is carried out so that the excitation current component command value 23 is matched with the excitation current component signal 15a through the operation of excitation current control circuit 24, the excitation voltage command value 24a increases. Similarly, since control is carried out in the torque current control circuit 29 so that the torque current component command value 27a is matched with the torque current element signal 15b, the torque voltage component command value 29a also increases.
Thereby, there rises a problem that the primary voltage command values 30a-30c increase, a current of the PWM inverter 5, and thus the current of the 3-phase induction motor 6, increase according to a time constant and finally the transistors within the PWM inverter 5 break down because such current exceeds the rated current of the transistors.
To solve these problems, a protection apparatus as disclosed in Japanese Laid-open Patent No. 61-123698 has been proposed. Namely, when a detected current of the current detector for detecting an output current of a inverter becomes lower than a preset value, operation of the inverter is inhibited by the operation inhibiting means of an inverter control apparatus and thereby a fault of transistor in a an inverter can be detected quickly and thereby a safer motor driving system can be obtained.
However, this prior art apparatus detects a transistor fault of the inverter at an early stage by stopping the operation of the apparatus when a detected current of a current detector for detecting an output current of the inverter becomes lower than a preset value, but simultaneously causes a problem in that it requires complicated circuits such as rectifying circuits, adding circuits, and discriminating circuits as indicated in FIG. 3 of the Japanese Laid-open Patent No. 61-123698 in order to process AC waveforms including ripples and also exhibits difficulty in the setting of values.
Moreover, the apparatus of the prior art further causes a problem that since a detected current value of the current detector becomes lower than a preset value for a certain period of time immediately after apply application of an operation signal because of the time constant of the motor motor which must elapse before the current can rise, the motor may stop immediately after reception of the operation signal because the detected current falls in the operation inhibit region.